The invention relates to an image pick-up apparatus, including a first and a second image sensor for converting a first and a second subimage into a first and a second electronic sub-image signal, respectively, and also a combination unit for deriving an electronic image signal for a composite image from the first and second electronic sub-image signals. The invention also relates to an X-ray examination apparatus including an X-ray source for irradiating an object by means of an X-ray beam in order to form an X-ray image, an X-ray detector for deriving an optical image from the X-ray image, and an image splitter for deriving a first and a second sub-image from the optical image. The invention also relates to a method of composing a composite image from a first sub-image picked up by a first image sensor and a second sub-image picked up by a second image sensor.
2. Description of the Related Art
An image pick-up apparatus and a method of this kind are known from German Offenlegungsschrift DE 33 15 882.
The known image pick-up apparatus forms part of an X-ray examination apparatus. The X-ray examination apparatus comprises an X-ray source and an X-ray image intensifier whereto the image pick-up apparatus is optically coupled. An X-ray image is formed of an object, for example a patient to be radiologically examined, arranged between the X-ray source and the X-ray image intensifier, by irradiating the object by means of an X-ray beam emitted by the X-ray source. The X-ray image is formed on an entrance screen of the X-ray image intensifier and converted into an optical image on an exit window of the X-ray image intensifier. The optical image is picked up by the image pick-up apparatus and converted into an electronic image signal.
The known image pick-up apparatus comprises two solid-state image sensors, each of which comprises a plurality of photosensitive elements. The image sensors are arranged in such a manner that pixels of the optical image which are picked up by one image sensor are situated in the intermediate spaces between the photosensitive elements of the other sensor. Each image sensor picks up a respective sub-image and applies a respective sub-image signal to the combination unit which forms an image signal for the composite image from said sub-image signals. The composite image is composed of image lines of the sub-images picked up by the individual sensors, i.e. so that image lines of one image sensor constitute the odd image lines in the composite image and image lines of the other image sensor constitute the even image lines in the composite image. In the direction transversely of the image lines the composite image has a spatial resolution which has approximately been doubled relative to the sub-images picked up by the individual sensors.
The known image pick-up apparatus has a drawback in that, even when the optical image is of uniform brightness, image lines from different image sensors may have different brightness values in the composite image. The differences may be due inter alia to the fact that the individual image sensors receive light from the exit window via different optical paths in which differences in light attenuation occur and/or that the sensitivities of the image sensors are not exactly equal. Such differences cause disturbances in the composite image. The composite image may exhibit, for example a streaky pattern which is not at all related to the image information in the optical image but is caused by unequal sensitivities of the image sensors.
The light beam emitted by the exit window is split into two sub-beams by a partly transparent reflector. Because the degree of transmission and reflection of this reflector is dependent on the angle of incidence of the incident light, a light intensity variation occurs within the sub-beams if different angles of incidence occur in the light beam. The image sensors convert this light intensity variation into variations of the brightness values in the sub-image signals. These variations in brightness values differ for the various sub-image signals and, if no further steps are taken, said different variations cause complex disturbances in the composite image. Such disturbances further degrade the image quality of the composite image. The disturbances are notably detrimental to the diagnostic quality of the composite image, because small details in the X-ray image may be lost in the complex brightness variations of the disturbances in the composite image. Particularly these small details in the X-ray image, however, may be of prime importance in establishing a correct diagnosis.
It is an object of the invention to provide an image pick-up apparatus enabling correction of variations of brightness values of the sub-images in order to counteract disturbances in the composite image.
To achieve this, a method in accordance with the invention is characterized in that the image pick-up apparatus comprises a correction unit for correcting brightness values, comprising a multiplier for multiplying brightness values of an image column of the first sub-image by a column gain factor and brightness values of an image line of the second sub-image by a line gain factor.
The image pick-up apparatus of the invention is intended notably for picking up sub-images, each of which consists of an image consisting of a raster of pixels. Pixels in one direction in the image constitute image lines and pixels in a direction transversely of the direction of the image lines constitute image columns. Each pixel thus forms part of one image line and one image column. The horizontal direction is customarily taken as the direction for the image lines and the vertical direction as the direction for the image columns, but this convention is not essential for the invention. In the scope of the present invention it is to be noted that, evidently, multiplication by a factor can be performed equally well by division by the reciprocal factor or by repeated addition.
In the composite image, composed by way of the corrected brightness values, the differences between brightness variations in the individual sub-images, in as far as these differences do not relate to image information, are compensated by a suitable choice of the values of column and line gain factors. As a result, disturbances in the composite image are counteracted and the diagnostic quality of the composite image is improved. When the invention is used in such a manner that the pixels of one sub-image are corrected per image line and those of the other sub-image per image column, comparatively few values will be required for these column and line gain factors, i.e. approximately the sum of the numbers of image lines and image columns, so that a memory of moderate capacity already suffices when predetermined column and line gain factors are stored. Even when separate sets of values of column and line gain factors are required for different circumstances in which an image is picked up, or separate settings of the image pick-up apparatus, the number of values is sufficiently limited so that it is practically feasible to fetch the values from a memory.
Because individual brightness values in the composite image need only be multiplied by a single column or line gain factor, an image pick-up apparatus in accordance with the invention requires only a single multiplier so as to form corrected brightness values.
It is another object of the invention to provide an X-ray examination apparatus for forming a composite image of high spatial resolution from sub-images derived from an X-ray image, said X-ray examination apparatus enabling correction of brightness values of the composite image in order to counteract disturbances in the composite image.
An X-ray examination apparatus comprising an X-ray source for irradiating an object by means of an X-ray beam in order to form an X-ray image, an X-ray detector for deriving an optical image from the X-ray image, and an image splitter for deriving a first and a second sub-image from the optical image, is characterized in accordance with the invention in that the X-ray examination apparatus comprises an image pick-up apparatus as claimed in claim 1 for deriving an electronic image signal of a composite image with corrected brightness values from the sub-images.
An X-ray examination apparatus provided with an image pick-up apparatus in accordance with the invention offers the advantage that an X-ray image is converted into an electronic image signal for a composite image of high diagnostic quality. Such high diagnostic quality is achieved in that the composite image has a high spatial resolution and contains few disturbances, so that small details of little contrast can be suitably reproduced, for example on a monitor. The radiologist can thus more readily notice small details, such as a tumor in an initial pathological stage.
The values of the column and line gain factors available are inter alia dependent on settings of the X-ray examination apparatus. These settings concern, for example the setting of the anode current and the high voltage of the X-ray source or the setting of a diaphragm for controlling the light intensity on the image sensors. In order to achieve accurate correction for various of such settings, a set of values for column and line gain factors is available for individual settings; for example, these sets of values are stored in a memory. Because only comparatively few column and gain factors, i.e. only approximately 2000, are required for a comparatively large composite image of approximately 1000.times.1000 pixels, sets of values for even numerous settings of the X-ray examination apparatus are available without requiring an excessively large and expensive memory.
It is another object of the invention to provide a method of forming a composite image from a first and a second sub-image and for supplying corrected brightness values for the composite image in order to counteract disturbances in the composite image.
A method of composing a composite image from a first sub-image picked up by a first image sensor and a second sub-image picked up by a second image sensor in accordance with the invention is characterized in that corrected brightness values are formed by multiplying brightness values of the first sub-image by a column gain factor which is variable per image column and by multiplying brightness values of the second sub-image by a line gain factor which is variable per image line, the composite image being formed by means of image lines with corrected brightness values of the first sub-image alternating with image lines with corrected brightness values of the second sub-image.
In order to counteract disturbances in the composite image, the column and line gain factors are determined so that the difference in the variation of the brightness in the individual sub-images is compensated in the composite image with corrected brightness values in as far as this difference does not relate to image information. Because individual brightness values need be multiplied by only a single column or line gain factor, the number of arithmetical operations is limited to a single multiplication per pixel, so that the method of the invention requires only little computing time.
A further preferred implementation of a method of the invention is characterized in that the column and line gain factors are derived from brightness values of a first and a second reference image picked up by the first image sensor and the second image sensor, respectively.
The values of column and line gain factors are essentially independent of the image information of the sub-images. Consequently, the values of column and line gain factors can be derived from the reference images so as to be stored in a memory. The values stored are then available to execute the multiplications of brightness values at a later instant. A single first and second reference image suffice to determine column and line gain factors for the correction of brightness values of many different composite images. The reference images may be special-purpose images wherefrom column and line gain factors are derived whereby accurately corrected composite images can be obtained in many different circumstances. The first and second reference images may furthermore be images containing the same image information.
A further preferred implementation of a method in accordance with the invention is characterized in that column gain factors for individual image columns are formed as the quotient of mean brightnesses in corresponding image columns in the first reference image and the second reference image, respectively, line gain factors for individual image lines being formed as the quotient of mean brightnesses in corresponding image lines in the second reference image and the first reference image, respectively.
Corresponding image lines and columns are to be understood to mean image lines and columns which contain substantially the same image information. When the composite image is formed by image lines of the first sub-image alternating with image lines of the second sub-image, a pair of corresponding image columns of the first and the second sub-image together constitute an image column of the composite image. A pair of corresponding image lines of the first and the second sub-image then constitute neighbouring image lines in the composite image. Because the column and line gain factors are derived from mean values of brightness values, the effect of noise is reduced. Such a mean value may be the sum of a number of brightness values, divided by their number or not, or also a weighted mean value.
A further preferred implementation of a method in accordance with the invention is characterized in that the column and/or line gain factors are derived from said mean brightness values by means of a bisection method.
The bisection method for determining the column gain factor for one of the image columns in the composite image involves determination of the difference between a first and a second mean value of the brightness in an image column of the first and the second image, respectively. Subsequently, a random first gain value is chosen whereby brightness values of the second sub-image are multiplied, the second mean value being subsequently determined therefrom. Subsequently, the sign of the difference between the first and the second mean values is determined. A second gain value is then searched, for example by trial and error, the sign of the difference between the first and the second mean values opposing that of the first gain value. Subsequently, for a third gain value, being between the first and the second gain value, the sign of the difference between the first and the second mean values is determined again. Subsequently, a fourth gain value is determined which is between the first and the third gain value or between the third and the second gain value, depending on the sign of the third gain value. By iteration, therefore, the gain value is found for which the difference between the first and the second mean value substantially disappears; this gain value is the column gain factor searched. The same bisection method, be it applied to the difference between mean brightness values of image lines of the first and the second sub-image and on the basis of amplification of brightness values of the first sub-image, produces the line gain factor searched for one of the image lines in the composite image.
An advantage of this bisection method consists in that only a small number of simple arithmetical operations is required for accurate determination of the column and line gain factors searched. Furthermore, no separate, complex control system is required for keeping the amplifiers accurately adjusted to required absolute gain factors.
A further preferred implementation of a method in accordance with the invention is characterized in that said column and line gain factors are stored in a memory and said column and line gain factors are fetched from the memory for the correction of brightness values of the first and the second sub-image.
The method of the invention for the correction of brightness values requires no more values for column and line gain factors than the sum of the numbers of image lines and image columns of a sub-image. As the number of pixels in the composite image increases, the number of values required, therefore, increases only slowly, i.e. approximately as the square root. When the values for the column and line gain factors are stored in a memory, a comparatively small memory capacity already suffices even for large composite images. For example, a memory capacity amounting to a few thousands of values suffices for a composite image comprising millions of pixels. Because only a comparatively small number of column and line gain factors need be fetched, the method takes little time even in the case of a large composite image.
A further preferred implementation of a method in accordance with the invention is characterized in that the first and second sub-images are used as the first and the second reference image, respectively, for calculation of the column and line gain factors, the column and line gain factors being calculated from brightness values of the first and a second sub-image. It is also possible to use the individual sub-images themselves, as picked up by the individual image sensors, as reference images. The correction of brightness values for the composite image is then very accurate for individual circumstances in which sub-images are picked up and processed so as to form a composite image, because the values thus determined provide optimum correction which is specific of the instantaneous sub-images.
Because column and line gain factors need be determined again for each subsequent composite image, more arithmetical operations are then required than when the column and line gain factors are derived from separate reference images.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments and implementations described hereinafter and with reference to the accompanying drawing.